Carrier transmission system



July 2, 1929.

E. L. NORTON CARRIER TRANSMISSION SYSTEM Filed July 24' 1925 UEQ QS. f

p Patented July 2, 1929.

UNITED STATES PATENT oFFicE.

EDWARD L. NORTON, OF NfW YORK, N. Y., ASSIGNOR TO WESTERN ELECTRIC COM- rm, mconona'rnn, 'or' NEW Yonx,

N. Y., A CORPORATION OF NEW YORK.

'emma 'rnANsMIssIoN SYSTEM.

Application led July 24, 1925. Serial No. 45,791.

This invention relates to wave transmission systems for signaling and kindred purposes and particularly to terminal \apparatus for such systems.

An object ofthe invention is to enable the y sary for changing from one condition to the` other. A

The carrier terminal unit of the invention is of the single side-band carrier sup ression type. Novel apparatus is provided or combining electrical carrier oscillationsand message currents to produce mechanical vibrations comprising upper and lower side-band components, the carrier being suppressed.V One of the mechanical side-bands thus produced is selec-tively transmitted by a mechanical filter and is then converted into a corresponding frequency band of electrical energy by a suitable electro-mechanical motor element. This frequency band is then selectivea terminal station of a carrier transmission system in which the invention is embodied.

Referring to the drawing, the carrier terminal station illustrated comprises a plurality of carrier channels I, II and III, which .are associated with`the main line ML through the band filters BF BF2 a'nd BF3 respectivel Carrier channel I comprises a low pass filter SF a source of carrier waves C an electro-mechanical modulator-detector MD a mechanical filter MF an electro-mechanical gter element ME and a band pass filter The low pass filter SF, inserted between the telephone transmitting and receiving elements (not shown) and the modulator-detector MDnmay be of the sup ression type disclosed in the patent to Camplbell 1,493,600 dated May 13, 1924. This filter may have a cut-off, for example, in the neighborhood o f 2500 cycles so that it will pass the essential speech frequencies and suppress higher frequencies. The anti-resonant circuit 1 in the series arm of the filter SF, may be tuned to give maximum attenuation at the carrier frequency supplied by the source 0 so as to prevent the carrier current from backing up into the telephone circuit. v

The electro-mechanical modulator=detector MD, consists of a fixed coil 2 (which may comprise two portions as shown in the drawv ing) and a movable coil 3, which is mounted so as to turn on an axis in its plane in a manner similar to a motor armature. The fixed coil 2 and the movable coil 3 are so mounted relative to each other that the interaction of the fields of the currents in the two coils will cause movement of the movable coil 3. To explain the operation of the system it will be assumed that the carrier source C, in channel I supplies current of 8000 cycles to the selective circuit including capacity 4 and fixed coil 2, this selective circuit being tuned to that frequency tol keep out extraneous frcquencies. The anti-resonant circuit 1 in the series arm of filter SF, then is tuned to give maximum attenuation at 8000 cycles. The movable coil 3 is mounted on a shaft 5 which is supported rigidly, and is made of the proper elasticity so that it will resonate mechanically with the moving coil 3 at the earrier frequency, in this case at 8000 cycles.

The operation of the apparatus in channel I will be first described. Voice currents passed by the low pass filter SF, are supplied to the movable coil 3 of the modulator MD,. The resultant force on the movable -coil 3 due to the interaction of the carrier and voice current fields is proportional to the products of the currents in the two coils. If the voice current in the movable coil '=I sin wt and thc carrier current in Lne fixed coil UZIO sin wCt, where aF-2W times the voice frequency and w=-1r times the carrier frequency then the mechanical torque F on the movable coil will be proportional to the product of the two currents F oo iva',- co [vlc sin on? sin out,

From this, it is seen that the mechanical torque comprises only components representing the sum and difference of the carrier and voice frequencies and has no components of carrier frequency, voice frequency, twice the voice frequency or twice the carrier frequency as in other modulators which are based on the square law.

In the particular case being described, the two components obtained comprise an upper side-band of 80GO-10,500 cycles, and a lower side-band, 5500-8000 cycles, the carrier being suppressed. When the movable coil 3 moves the resulting torsional mechanical vibrations are transmitted to the mechanical filter MF1, which may be designed to suppress one mechanical side-band, for example, the lower side-band 5500-8000 cycles, and to transmit the other side-band.

The mechanical filter MF, may be of any suitable type but is preferably of the type shown in the drawing, which is disclosed in applicants copending application Serial No. 752,082, filed November 25, 1924. As described in that application, this mechanical filter may consist of a circular loop or a plurality of circular loops of metal rod, the radius of curvature of the loops, and the material and diameter of the rod being selected so that the filter transmits with practically negligible attenuation, torsional mechanical vibrations transmitted thereto of all frequencies higher than a lower limiting value while substantially attenuating mechanical vibrations of all frequencies lying on the other side of the limiting frequency.

The mechanical vibrations comprising the upper side-band component, 80GO-10,500 cycles, are transmitted by the mechanical filter MF1 to an electro-mechanical motor element ME? vThis motor element may be of any suitable type but is preferably of the type disclosed in the copendingr application of I-I. C. Harrison, Serial No. 678,935, filed December 6, 1923, which type is designed to convert a frequency band of mechanical energy supplied thereto into a corresponding frequency band of electrical energy. In this particular case, the motor element MF)1 is designed to convert the mechanical side-ba nd of 8000-10,- 500 cycles to the corresponding electrical. sideband of 80GO-10,500 cycles. This frequency band is selected by the band filter BF1 and transmitted over the line ML.

Low frequency telephone currents from the regular voice channel TL may be independently placed on the line ML through a low pass filter 6, which is so designed as to pass these low frequencies but to prevent the high-v er carrier frequencies from entering the voice channel.

The operation of the circuit I in detecting messages received over the line ML from the distant station is exactly the reverse of that described for transmission, the same apparatus being adapted for both transmittingl and receiving messages. This operation may be described as follows: The message modulated carrier wave comprising the side-band component, SOOO-10,500 cycles, received over the line ML is selected by the band filter BF1 and transmitted to the electro-mechanical motor element MED which converts its electrical energy into mechanical energy of corresponding frequencies. The mechanical sideband of SOOO-10,500 cycles thereby produced is selectively transmitted by the mechanical filter MF1 and causes corresponding movements of the movable coil 3 of the modulatordetector MB1. The movement of coil 3 in the magnetic field around fixed coil 2 produced by the carrier current of 8000 cycles from source C, will cause the speech currents to be demodulated and induced in movable coil 3. The speech currents are then transmitted from movable coil 3 through the low pass filter SF1 to the telephone receiving apparatus (not shown).

The two-way operation of the system for a single carrier channel has been described above. If it is desired to transmit a plurality of messages at the same time over the line M'L, additional channels II, III may be utilized. The apparatus in these channels, except for the frequency of the carrier sources and the transmission ranges of the mechanical and electrical filters, is similar to the similarly designated apparatus in channel I and operates in a similar manner, as described above. For example, as indicated in the drawing, the frequencies of the carrier sources C2 and C3,v the transmission ranges of the mechanical filters MF2 and MFa, and of the band filters BF2 and BF3 may be so selected that in addition to the ordinary low frequency telephone currents from line TL, and the single side-band, SOOO-10,500 cycles, from carrier channel I, message modulated carrier waves comprising frequency bands of 11,000-13,500 cycles and 14,500-17,000 cycles, or any other suitable frequencies, respectively, may be transmitted over the line ML Without interference. The particular frequencies indicated in the drawing are chosen for dcscriptive purposes only and are not to be considered as limiting the invention.

In case only one carrier channel is desired, the electrical hand filter BF1 may not be necessary, as the mechanical filter MF1 will separate satisfactorily the voice frequencies and the single carrier side-band.

The low pass filter 6 and the band filters BFl, BF2, BF3 may be designed in accordance with the principles set forth in the United States patent to Campbell, No. 1,227,- 113, issued May 22, 1917. f

The definite values that the constants of the electromechanical modulator-detector system described above should have in order that a maximum efficiency over a predetermined band of frequencies may be obtained may be determined from the following mathematical analysis. In the analysis, the following symbols for the various constants of the system will be employed:

N=Number of turns on the moving coil 3.

a=Area of moving coil 3.

B=Flux density of carrier field from coil 2 through moving coil 3.

L=Inductance of moving coil 3.

m=Moment of inertia of moving coil 3.

S=The elasticity of the shaft 5 on which the moving coil 3 is mounted.

Z1=The impedance of the voice fre uency electrical circuit (that including SF1, F2 or SF3) at the terminals connecting it to MDI,

Z2=Mechanical impedance of mechanical circuit of the structure including coil 3 and shaft 5.

Z.=Ima e impedance of voice frequency electrical circuit referred to above.

Zb=Image impedance of mechanical circuit includmg coil 3 and shaft 5.

P= The transfer constant. 6=Angle which lane of moving coil 3 makes with the carrier field. (It is assumed that 0 is small so that sin @#0) 0=time despectively, as follows:

zli+Na Bo =E (1) zza-vanzelf7 (e) It is assumed that cach of these circuits has a high impedance to frequencies outside of B=B sin wat (3) 0=sin wzt (4) l From Equations 1 to 5 the following equations are derived If the mechanical circuit is considered as closed through an impedance Zb with the torque F=O, and the electrical circuit as closed through an impedance Za with E=O, then the structure may be treated as a filter with image impedances Za and Zh respectively, and with a transfer constant P. The theory of image impedanees and transfer constants of networks is discussed fully on pages 74 to 77 of Transmission Circuits for Telephonie Communication by K. S. Johnson (1924 edition) and need not be discussed M=NaB\/1 -i-ll (11) wz Z1 will be taken as an inductive reactance 7 c and Z2 as the reactance of a resonant mass and elasticity resonating at w, /21r. Then Z2 Jsm( wzwa (12) Substituting the values for Z1, Z2 and M in Equation (10) it is apparent that the structure, in so far as the voice frequencies are concerned, has a low pass filter image impedance, with a cut-off frequency given by As Zl now becomes the mid-series impedan'ceof a low pass filter, other sections may be joined to it, and the complete. impedance of the moving coil 3 may be taken as 2Z1, the inductance of the coil being L. l

The structure may then be designed as follows:

L, the inductance of the moving coil 3, is considered as a full series inductanee of an electrical low pass filter and is fixed by the .ordinary design relations for such structures from the impedance of thel electric circuit and the eut-off frequency desired.

The maximum carrier fiux density required, then; is fixed by Equation 13 and is equal to:

S=w32m (15) Although the invention has been specifically described with reference to a carrier telephone system, it is to be understood that it is not so limited but it may be used as well in a carrier telegraph system. It is to be understood also that the invention is not limited in scope to the particular embodiment shown and described or even to wire transmission, but only by the appended claims.

What is claimed is: y

1. A modulating system comprising a source of electric currents of high frequency,

. a source of electric currents of low frequency,

a mechanical element, electromagnetic means to combine sald high frequency currents and said low frequency currents to produce vibrations of said mechanical element comprising components representing the sum and difference of said high and said low frequencies, said means comprising a fixed coil and a movable coil, said high frequency currents being supplied to one of the coils and said low frequency currents being supplied to the other of said coils, a vibration responsive device, and means for selectively transmitting one of said vibration components to control said device.

2. In a signaling system, a source of carrier current, a source of signaling currents, a movable mechanical element in the magnetic fields of said carrier current and said signaling currents, and means to combine the magnetic fields of said carrier current and said signaling currents so as to produce mechanical vibrations of said mechanica-l element having components representin the sum and difference of the frequencies o said carrier current and said signaling currents.

3. A translating system comprising a source of electric currents of high frequency, a source of electric currents of low frequency, means tocombine said high frequency currents and said low frequency currents to pro- .duce mechanical vibrations comprising components representing the sum and difference of said high and low frequencies, and a mechanical filter for selectively transmitting one of said components of said mechanical vibrations.

4. In a signaling system, a source of carrier waves, a source of signaling waves, electromagnetic modulating means to combine said carrier wave and said signaling waves to 7 produce mechanical vibrations having side bandcomponents representing the sum and difference of the frequencies of said carrier waves and said signaling waves, and a mechanical filter associated therewith for selectively transmitting one of said mechanical side-band components and suppressing the other of said side-band components.

5. A system for combining electrical currents of different frequencies to obtain mechanical vibrations having components representing the sum and difference of the frequencies of the two currents, said system comprising a fixed coil supplied with currents of one frequency, a movable coil supplied with eurrent of the other frequency, and a mechanical element responsive to the movements of said movable coil caused by the interaction of the fields produced by the currents fiowing in said fixed coil and in said movable coil.

6. In a signaling system, a source of carrier current, a source of signaling currents, a mechanical element, and means to combine said carrier current and said signaling currents so as to produce mechanical vibrations of said mechanical element having components representing the sum and difference of the frequencies of said carrier current and said signaling current, said means comprising a fixed coil supplied with said carrier current and a movable coil supplied with said signaling currents, said movable c oil bein mounted so as to be free to move in the fiel 4produced by the carrier current fiowing in said fixed coil, said mechanical element being mechanically responsive to the movement of said movable coil.

7. A modulating system comprising a fixed coil, a movable coil, means for supplying carrier current to said fixed coil, means for supplying modulating current to said movable coil, said fixed coil and said movable coil being so mounted with respect to each other that the interaction of the fields produced by the currents flowing in the two coils will cause movement of said movable coil, a mechanical element responsive mechanically to movements of said movable coil, said mechanical 'element bein of such elasticity that it will resonate mec anically with said moving coil at the frequency of said carrier current.

8. A modulating or demodulating system having elements comprising a fixed coil, a movable coil, a terminal circuit connected to said movable coil, means for supplying carrier current to said fixed coil, said movable coil being mounted so as to be free to move in the field produced by said carrier current, and a mechanical vibration device associated with said movable coil, all of said elements being so arranged and proportioned that the system converts energy in either direction between electrical energy in said terminal circuit and mechanical vibratory energy in said movable coil and vibration device substantially without attenuation over a predetermined range of frequencies and with substantial attenuation outside that range of frequencies.

9. A signaling system comprising a carrier frequency line, a low frequency terminal circuit and a two-way repeating means for translating low frequency signals in said terminal circuit into signal-controlled carrier waves for transmission over said carrier frequency line and for translating signal-controlled carrier waves from said line into low frequency signals in said terminal circuit, comprising means for translating the signals or the signal-controlled waves into mechanical vibratory energy, a mechanical filter for selectively transmitting mechanical vibrations of the frequency band of the vibrations so translated, and means for retranslating the selectively transmitted mechanical vibrations into electrical waves, one of said translating means being a modulator-de-modulator for shifting the frequency level of the translated waves from the signal-frequency level to the carrier frequency level and vice versa.

10. In combination in a carrier signaling system, a transmission line and a two-way terminal station comprising a common means for generating and transmitting over said line, a carrier modulated wave consisting of either one of two side bands of frequencies,

` `and for demodulating incoming carrier-modulated waves having frequencies corresponding to those of the transmitted band, said comm'on means comprising an electro-mechanical modulator-demodulator as in claim 6, amechanical filter associated therewith for selectively transmitting in either direction mechanical vibrations representing said side band offrequencies, and an electro-mechanical motor element associating said mechanical filter with said line, said motor element being adapted for converting the mechanical energy received from said mechanical filter into electrical energy of corresponding frequencies, and for converting the electrical side band received over said line into mechanical vibrations of corresponding frequencies.

11. A carrier telephone system including a speech-frequency circuit, a carrier frequency transmission line, and a repeating means therebetween comprising means for translating currents representing speech into mechanical vibrations representing speech-modulated carrier Waves, a mechanical wave filter for selectively transmitting said mechanical vibrations, and means for translating the selectively transmitted vibrations into electrical jy waves and impressing them on said carrier frequency transmission line.

12. A carrier telephone system including a speech frequency circuit, a carrier frequency transmission line, and a repeating means therebetween comprising means for translating speech modulated carrier currents into mechanical vibrations of corresponding frequency, a mechanical wave filter for selectively transmitting said mechanical vibrations, and means for translating the selectively transmitted mechanical vibrations into speech currents and impressing them on said speech-frequency circuit. 13. In a modulating system, a source of carrier waves, a source of signaling waves, a movable mechanical element in the magnetic fields of said carrier waves and said signaling waves, and means for combining the magnetic fields of said carrier waves and said signaling waves to produce mechanical vibrations of said mechanical element representing said carrier waves modulated with said signaling waves.

In witness whereof, I hereunto subscribe my name this 18th day of July A..D., 1925.

EDWARD L. NORTON. 

